Electrowetting technology is based on modification of an energy balance between on one hand surface tension forces of liquids and wetting properties of a solid surface, and on the other hand electrostatic forces induced by an applied voltage over a capacitor arrangement comprising said boundary layer.
An electrowetting optical element or cell, further referred to as electrowetting element, according to the state of the art may from bottom to top be comprised of respectively a first electrode layer stack comprising a substrate, a first electrode layer, an electrically insulating hydrophobic layer or an insulating layer having a hydrophobic surface on a side opposite to the first electrode layer, for interfacing to a polar liquid and a non-polar liquid immiscible with each other, and a second electrode layer stack, comprising a second electrode which is electrically in contact with the polar liquid and a superstrate for supporting the second electrode layer. The second electrode layer has a hydrophobicity that is lower than the hydrophobic interface surface of the first electrode layer stack. This causes the non-polar liquid to be present near the hydrophobic surface of the first electrode stack and the polar liquid to be present near the less hydrophobic interface surface of the second electrode stack. Pixel walls attached to the first electrode layer stack and extending from the first electrode layer stack towards the second electrode layer stack, form a containment space between the first and second electrode stacks and the pixel walls. The pixel walls thus form a barrier for the non-polar liquid between the electrowetting cell and adjacent electrowetting cells.
An electrowetting element can thus form a picture element or pixel. A plurality of electrically controlled electrowetting elements can together form a display or part thereof comprising pixels, which can be used for displaying arbitrary images by appropriately controlling the electrowetting elements forming the display. Electrowetting elements can have arbitrary shapes determined by the shape of the pixel walls, such that displays can be manufactured for specific purposes.
An electrowetting element is mainly transparent, except for the non-polar liquid in each of the pixels formed by the pixel walls. The non-polar liquid is often non-transparent or has a low optical transmission coefficient. The transmission coefficient of the non-polar liquid typically depends on the application of the electrowetting element. In a colour display, an electrowetting element comprising coloured non-polar liquids may be used.
Electrowetting elements can be applied in a transmissive implementation, using for example backlighting to light-up the display screen. In another implementation the electrowetting elements may be applied in a reflective set-up, for example by providing a (specular or diffuse) reflective surface at one of the electrode layers.
The principles of operation of an electrowetting element are as follows. In an unpowered state, i.e. when no voltage is applied over the first and second electrode, the lowest energetic state of the system is where the non-polar liquid forms a boundary layer between the polar liquid and the hydrophobic surface of the insulating layer. This is because the polar liquid is repelled by the hydrophobic layer. The poor transmissibility of the non-polar liquid then forms an obstruction to light that penetrates the system.
When a voltage is applied over the electrodes, the lowest energetic state of the system becomes the situation wherein the (poorly conductive or insulating) non-polar liquid is pushed aside by the (conductive) polar liquid, and the polar liquid thereby being in direct contact with the insulating hydrophobic layer. Note that the voltage must be large enough for the electrostatic forces to overcome the repellent and surface tension forces that separate the polar liquid from the hydrophobic surface. In this situation, light that penetrates the system has rather unobstructed access to the insulating hydrophobic layer because of the well transmissibility of the polar liquid and the non-polar liquid being pushed aside. In the powered up state, when voltage is applied over the electrodes, the electrowetting element is thus transmissive. This working principle is used in electrowetting type displays and screens.
Electrowetting elements according to the state of the art exhibit poor adhesion of the pixel wall to the hydrophobic surface, causing the pixel walls to deteriorate, thereby destroying the electrowetting element and it's surrounding electrowetting elements which are defined by the pixel walls. This is due to the fact that the hydrophobic surface has a very low contact angle hysteresis and low surface tension which makes it extremely difficult to apply an uniform coating of photoresist from which the pixel walls are formed. Furthermore the adhesion of the photoresist to the hydrophobic surface is very poor, thus resulting in poor adhesion and deterioration of the pixel walls. This problem may already occur during manufacturing of the electrowetting elements, when pixel walls are created. As a result, the yield of commercial manufacturing processes is lowered and in addition the technical life of electrowetting displays is shortened.
In the art, adhesion of the pixel walls can be improved by performing a surface modification of the hydrophobic surface for making this surface less hydrophobic. After manufacturing of the pixels walls the surface is returned to a hydrophobic state by an annealing step. However the return to a hydrophobic state with minimal contact angle hysteresis is often not perfect causing imperfect spreading of the oil after switching to the unpowered state. Such a method thus leads to problems with the opening and closing of the pixels during switching operation.